Elsevier

Talanta

Volume 178, 1 February 2018, Pages 9-14
Talanta

Direct detection of fluoride ions in aquatic samples by surface-enhanced Raman scattering

https://doi.org/10.1016/j.talanta.2017.08.101Get rights and content

Highlights

  • We demonstrated a novel method for detecting fluoride in water based on SERS.

  • A considerable limit is as low as 1.0 μmol L−1 (0.018 ppm).

  • It is a practical and effective method for fluoride detection in toothpaste.

Abstract

Given the strong hydration propensity of fluoride ions, it is difficult to detect fluoride, especially inorganic fluoride, in aqueous samples. Resolving the issue of fluoride detection in aqueous samples is a scientific undertaking of great practical significance. Herein, we propose a new method for the sensitive and selective detection of fluoride in aqueous samples without the addition of organic solvents. The method involves surface-enhanced Raman spectroscopy using 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) compounds and Ag nanoparticles. The method is based on a diketopyrrolopyrrole compound linked to 1-butyl iodide (DPP1), which can sense fluoride sensitively and selectively. When DPP1 was combined with Ag NPs and reacted with tetrabutylammonium fluoride or inorganic fluoride in aqueous samples, an obvious Raman enhancement was obtained at the excitation wavelength of 633 nm. This response arises because the introduction of fluoride anions into the system changes the molecular orientation of DPP1 on the Ag NP substrate from horizontal to vertical, inducing a signal enhancement in the Raman spectrum. This system can detect inorganic fluoride at concentrations as low as 1.0 μmol L−1 (0.018 ppm), which is far below the public health service recommended levels for drinking water (0.7–1.2 ppm). Furthermore, using the proposed method, a linear response for fluoride in the concentration range of 1.0 × 10−3–1.0 × 10−6 mol L−1 was obtained, which makes fluoride detection possible in practical samples, such as fluoride-containing toothpaste.

Graphical abstract

We have developed a significant, efficacious, simple method with less synthetic steps for fluoride detection in water by using incorporating a molecular probe (DPP1) with a SERS-active substrate. This method used SERS to solve the problem of fluoride detection in water, which dramatically reduced the synthesis work. In contrast to the traditional colorimetric sensor assays, this method has a good detection limit and excellent performance.

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Introduction

In recent years, many methods for sensing and recognizing anions have been developed [1], [2], [3], [4], [5], [6]. Fluoride, which is the smallest anion with the highest charge density, is an attractive target for anion detection owing to its application in diverse fields involving biological, medical, military, and technological processes [7], [8], [9], [10]. Excessive exposure to fluoride may cause collagen breakdown, rarefaction of bones, dental fluorosis, and neurological and metabolic dysfunctions [11], [12], [13], [14]. Fluoride is a candidate for prospective use in ion batteries [15] and may find utilization in the development of superconducting and hydrogen storage materials [16].

The widespread applications of fluoride have led to extensive fluoride contamination in the environment. Thus, processes and technologies for sensing fluoride have attracted much interest. Some of the current methods used for fluoride detection include the electrode method [17], 19F nuclear magnetic resonance (19F NMR) analysis, and chromatography [18]. However, these methods are complex and time-consuming when compared to optical chemsensors that can sense target molecules quickly and easily. Therefore, focusing on the development of an optical chemsensor for fluoride detection is a worthwhile pursuit.

In recent years, numerous fluorescence chemosensors have been developed for fluoride detection. Most reported fluorescent probes for fluoride detection are based on three interaction mechanisms: hydrogen bonding between fluoride and NH hydrogen [19], [20], [21], combination with boron compounds [22], [23], and fluoride-induced desilylation [24], [25], [26]. However, most of the probes designed on the basis of these three mechanisms require intensive synthesis procedures and the probe molecules are generally insoluble in water. The main probe molecules reported require organic solvents such as ethyl alcohol, acetone, or a mixture of an organic solvent and water [27], [28], [29], [30]. Surface-enhanced Raman scattering (SERS) is a powerful technique for anion sensing, and has been applied for sensing toxins [31], contaminants [32], [33], biomolecules [34], [35], etc. Moreover, SERS does not suffer from interference by water, which makes it beneficial for detecting target objects in aquatic samples. This method can provide molecular fingerprint information for the identification and quantification of target analytes, even at the single-molecule level, owing to the chemical and electromagnetic enhancement effects of the noble metal substrates for the analytes [36].

He et al. reported 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP)-based compounds combined with 1-butyl iodide (DPP1), which could sense fluoride (TBAF) selectively and sensitively in organic solvents but was ineffective for sensing fluoride (NaF) in aqueous samples [19]. DPP and its derivatives are brilliant red, strongly fluorescent, and are stable against light, weather, and heat [37], [38], [39]. Herein, DPP1 is selected as a probe molecule, combined with silver nanoparticles (Ag NPs), for the convenient and effective detection of fluoride in aqueous solution without the addition of organic solvents. The proposed method reduces the synthesis effort and facilitates the detection of fluoride in aqueous samples.

Section snippets

Materials and reagents

4-Bromobenzonitrile (99.0%), 1-butyl iodide (99.0%), potassium tert-butylate (98.0%), 2-methyl-2-butanal (99.0%), diisopropyl succinate, chromatographically pure tetrahydrofuran (THF), and tetrabutylammonium X (TBAX, X = F, Cl, Br, I) were obtained from Aladdin (Shanghai, China) and used as received without further purification. Poly (diallyldimethylammonium chloride) (PDDA: Mw = 200,000–350,000; 20 wt%) aqueous solution, silver nitrate (AgNO3, 99.0%), and sodium citrate (Na3C6H5O7, 99.8%) were

Fabrication of the SERS-based fluoride chip

The self-assembled SERS-active silver colloid substrate was obtained according to the previously reported method. The glass slide with hydroxyl groups was modified with the positively charged PDDA polymer (Fig. 1). Subsequently, the negatively charged silver colloid was assembled on the glass slide. The DPP1 molecules were then adsorbed on the above-mentioned substrate. The synthetic procedure [19] and nuclear magnetic resonance (NMR) spectrum of DPP1 are presented in Fig. S1 and Fig. S2,

Conclusion

In conclusion, we have developed a significant, efficacious, and simple method, with fewer synthetic steps, for fluoride detection in water by incorporating a molecular probe with a SERS-active substrate. This method uses SERS to solve the problem of fluoride detection in water, which dramatically reduced the synthesis work. The fingerprint information of DPP1 from the SERS profile can be used to monitor the molecular orientation between the molecule and substrates. In contrast with traditional

Acknowledgements

The National Natural Science Foundation (Grants 21327803, 21411140235) of the People's Republic of China.

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